The following USE changes are necessary to proceed:
#required by media-libs/mesa-8.0.4-r1, required by app-emulation/emul-linux-x86-opengl-20120520, required by app-emulation/emul-linux-x86-xlibs-20120520[opengl], required by x11-drivers/ati-drivers-12.6, required by @selected, required by @world (argument)
>=dev-libs/libxml2-2.8.0-r2 python

The following USE changes are necessary to proceed:
#required by media-libs/mesa-8.0.4-r1, required by app-emulation/emul-linux-x86-opengl-20120520, required by app-emulation/emul-linux-x86-xlibs-20120520[opengl], required by x11-drivers/ati-drivers-12.6, required by @selected, required by @world (argument)
>=dev-libs/libxml2-2.8.0-r2 python

You need to set the python use flag for libxml2:

Code:

# nano -w /etc/portage/package.use

dev-libs/libxml2 python

Quote:

/etc/portage/package.accept_keywords

Code:

>=dev-libs/libxml2-2.8.0-r2

This seems like a mistake. This would allow portage to pull in unstable versions of libxml2.